This invention relates to extreme purity aluminum and more particularly to an improved method for producing extreme purity aluminum.
Because of the growing awareness of the limitation or natural resources, particularly energy resources, considerable effort has been expanded to produce alternate sources. One such source which is considered to have exceptional long term potential to fulfill this need is the energy from a fusion nuclear reactor. However, because of the need to isolate or confine the radioactive media involved, considerable investigation is underway to develop materials for the reactor which will not subsequently present disposal problems. For example, if extreme purity aluminum were used in the reactor, the radioactivity of such material would be reduced by a factor of a million a few weeks after shutdown, provided the purity of the aluminum was sufficiently high. By comparison, if stainless steel were used for the same application, this reduction would take about 1000 years, obviously presenting difficult problems in disposing of such materials.
Another energy related field where extreme purity aluminum can be used to great advantage is stabilization of superconductors. In this application, the electrical energy is transferred at cryogenic temperatures, e.g. 4.degree. K, where the electrical resistance is very low. The use of extreme purity aluminum as a stabilizer is preferred in this application because of its very low resistivity, i.e. high conductivity at such low temperatures.
For example, aluminum having a purity of 99.9 wt.% would have an electrical conductivity factor at 4.degree. K of 20 times that of its room temperature value while a 99.999 wt.% aluminum would have a corresponding increase in conductivity of at least 1000 times and a 99.999 wt.% aluminum would have a conductivity factor at 4.degree. K of 5000 times its room temperature value. Thus, the total purity of the aluminum gives a reasonable indication of the conductivity at 4.degree. K. However, the concentration of certain critical impurities is more important. These critical impurities include titanium, vanadium, zirconium, chromium, manganese and iron. For example, the effect of chromium on low temperature conductivity is 20 times greater per ppm than copper--a relatively innocuous impurity as far as superconducting applications are concerned. Unfortunately, none of the prior art processes is effective in completely removing all of these critical impurities at reasonable costs.
For many years, purified aluminum was produced in an electrolytic cell having three liquid layers--two molten aluminum layers separated by a salt or electrolyte layer. The bottom or lower layer in the cell is the impure or aluminum-copper alloy layer and formed the anode of the cell and was purified by electrolytically transferring molten aluminum through the intermediate salt layer to the higher purity molten aluminum layer or cathode. Such cells, in various forms, are described in Hoopes U.S. Pat. No. 1,534,320; Hoopes U.S. Pat. No. 1,535,458; Hoopes U.S. Pat. No. 1,562,090 and Hulin U.S. Pat. No. 1,782,616, for example. This electrolytic cell, known to those skilled in the art as the Hoopes cell, is effective in reducing impurities such as manganese, chromium, titanium, vanadium, zirconium and gallium to a very low level. However, such a cell is less effective in lowering the concentration of impurities such as silicon, iron, copper and the like. That is, after passing aluminum to be purified through a Hoopes cell, significant amounts of silicon, iron and copper can be found in the high purity cathode layer, although at much lower concentrations than in the anode layer.
The prior art also discloses that high purity aluminum can be produced by several other methods; however, all of these methods taken individually can have serious drawbacks, especially when it is desired to produce large quantities of extreme purity aluminum at economically attractive costs. For example, zone refining, which can produce extreme purity aluminum, has the disadvantage that it can be difficult to scale to production quantities.
It is also known that certain impurities can be removed by adding boron to aluminum in the molten condition, thereby forming a boron-containing compound or complex having a higher density than the aluminum, resulting in the compound precipitating out. This process of purifying aluminum is taught by Stroup in U.S. Pat. No. 3,198,625 and described in an article by Russell et al entitled "A New Process to Produce High-Purity Aluminum" at pp. 1630 to 1633 of Vol. 239, Transaction of the Metallurgical Society of AIME (October 1967). However, as noted in the patent, while this process is particularly effective in removing titanium, vanadium, zirconium, and to a lesser degree chromium, it has substantially no effect on the removal of other common impurities such as iron, silicon, copper and the like.
Another method in the prior art used for the purification of aluminum is referred to as preferential or fractional crystallization. Such crystallization methods are disclosed by Jarrett et al in U.S. Pat. No. 3,211,547 and by Jacobs in U.S. Pat. No. 3,303,019 (both patents included herein by reference) and in the aforementioned Russell et al article. However, while the methods disclosed in these publications can result in fractions of very high purity aluminum, there also results, as disclosed by Jarrett, a fraction of relatively low economic value and at least one intermediate fraction, in respect to aluminum which is not widely variant from the starting material. Furthermore, this process does not remove elements such as titanium, zirconium, vanadium, manganese and chromium.
While each of the foregoing prior art processes is effective for the removal of certain impurities, none of the processes individually remove all of the undesirable impurities which should be removed for certain applications of extreme purity aluminum such as in the field of superconductors as previously discussed. Furthermore, each of the processes suffers economically--the fractional crystallization because of the low yield of high purity aluminum per kilogram of aluminum which must be heated to its melting point to permit such separations and the electrolytic purification because it does not effectively remove all impurities to a sufficiently low level.
The present invention solves the problems such as described in the prior art involving purification of aluminum by providing a process which produces extreme purity aluminum in an economical manner in large production quantities and in which process, for every pound of impure aluminum beneficiated, almost one pound of extreme purity aluminum is obtained. The cost of extreme purity aluminum produced in accordance with the present invention is quite low compared to conventional practices.